Available chlorine multi-stage bleaching of sulfite pulp



t ittlt Mahatma.

AVAILABLE CHLORINE MULTl-STAGE BLEACHING F SULFITE PULP William A.Stone, New Orleans, La., assignor to Olin Mathieson ChemicalCorporation, a corporation of Virginia No Drawing. Application March 26,1952, Serial No. 278,747

2 Claims. (Cl. 8-105) My invention relates to improvements in themultistage bleaching of sulfite cellulose pulps and more particularly toa bleaching operation employing a combination of chlorine dioxide,particularly in the form of a chlorite, and chlorine in the first orpreliminary bleaching stage.

In the conventional methods of multi-stage bleaching of sulfitecellulose pulps, chlorine has been acommonly used reagent. While thisreagent, used most frequently in conjunction with hypochlorites willproduce a satisfactory color, it tends to impair the strength of thesulfite pulp, the tendency being particularly marked in cases where ahigh degree of bleaching is required. The strong oxidizing power ofchlorine and the hypochlorites is apparently a chief cause of thisundesirable degradation, although direct reaction of the chlorine andsulfite pulp may be partially responsible. For example, although sulfitepulp has the advantage over sulfate pulp in that it is more readilybleached to a high whiteness and is thus more suitable for many gradesof paper and other cellulose products, the high whiteness is frequentlyobtained at the expense of a considerable loss of strength. For thisreason sulfate pulp is sometimes blended with sulfite pulp in spite ofthe sacrifice in color in order to obtain adequate strength.

In the bleaching of sulfite pulp many variations in procedure arepossible. However, the minimum operation ordinarily consists of threestages, the first of which is a chlorination stage in which the pulp istreated with chlorine water for a period of about one hour at an acidpH. Under these conditions the lignin is largely chlorinated due to thepresence in the acid solution of most of the chlorine as dissolvedelemental chlorine. Oxidation is avoided by the acidity of the solution,the pH normally being about 1.8 to 2.0, which restricts the formation ofhypochlorous acid, the oxidizing agent in the system. Furthermore, sincethe chlorination reaction is relatively rapid compared to the oxidationreaction, limiting the time of contact also assists in avoidingoxidation. In this preliminary or chlorination stage it is customary tosupply about 50 to 60 per cent of the chlorine demand of the pulp asdetermined by the permanganate number. The pulp consistency is about 1.5to 3 per cent and the preliminary bleach is carried out at thetemperature of the available water. The chlorinated pulp is extractedwith caustic, for example with 2 per cent caustic, at 120 to 140 F.using a 10 per cent pulp consistency. After washing, the bleach iscompleted by treatment with a hypochlorite under alkaline conditions.The resulting washed pulp has a brightness of about 80 to 82.

While chlorine dioxide and chlorites have been widely used in bleachingsulfite pulps to obtain high brightness without decrease in strength,tearing or fold resistance, their use has ordinarily been restricted bythe relatively high cost of chlorites as compared with chlorine tostages subsequent to the first or early chlorine bleaching stage wherethe principal advantages of the chlorine dioxide and chlorites areutilized to the maximum degree.

2 ,741,536 Patented Apr. 10, 1956 I have now found that a combinationchlorine dioxidechlorine bleach in the first or preliminary stage inbleaching a sulfite pulp permits the realization of substantialadvantages for the bleaching operation, particularly in chlorinesavings. For example, I have found that the combination chlorinedioxide-chlorine bleach unexpectedly reduces the total availablechlorine requirement of the pulp to a substantial extent, e. g. by 20 to50 per cent. In addition, equivalent or higher brightnesses, greaterstrength retention, easier and faster bleaching and less color reversionare obtained. Moreover, the reduction in acidity and lower temperaturein the preliminary chlorination stage avoids damage to the pulp by theacid and saves corrosion of the equipment besides making possible theelimination of the caustic extraction stage.

According to my multi-stage process a sulfite cellulose pulp in aqueousmedium is partially bleached in a preliminary stage with sufiicientchlorine dioxide and chlorine to supply about 50 to 60 per cent of thechlorine requirement of the pulp as determined by the permanganatenumber and then bleached in a subsequent stage with an aqueous solutionof a hypochlorite. The amount of the chlorine requirement of the pulpsupplied to the preliminary partial bleaching stage by the chlorinedioxide is about 50 to per cent on an equivalent available chlorinebasis. Advantageously, the pulp in aqueous medium is partially bleachedin the preliminary stage with a chlorite and chlorine at a pH of about2.5 to 2.7 at approximately room temperature.

In general, I have found that best results at the most economical levelare obtained when the amount of the chlorine requirement of the pulpsupplied to the preliminary partial bleaching stage by the chlorinedioxide is about 50 to 75 per cent on an equivalent available chlorinebasis. Particularly if a chlorite is used in the preliminary partialbleaching stage, in order to realize the advantages of my invention, itis important that more than sufficient chlorine be supplied than isrequired to activate the chlorite.

Although the following examples deal with the multistage bleaching of asulfite spruce pulp, it is to be understood that the examples are merelyillustrative of my invention. Preliminary conventional multi-stagebleachings of this sulfite spruce pulp were performed using 30 pounds ofchlorine per ton of pulp in the preliminary or chlorination stage, 0 to20 pounds of caustic per ton of pulp in the second or caustic extractionstage, and 30 to 34 pounds of available chlorine ar calcium hypochloriteper ton of pulp in the third or hypochlorite bleaching stage. Theresulting pulp had a brightness of from 78.0 to 83.0 depending on theamounts of caustic and hypochlorite used in the second and third stagesrespectively. For a brightness of 7879, no caustic and about 30 to 34pounds of hypochlorite per ton of pulp were used. Twenty pounds ofcaustic and 34 pounds of hypochlorite per ton of pulp were used when abrightness of 82-83 was desired. Strength retention of test sheets of8283 brightness was 94.8 per cent based on per cent Mullen values and100.5 per cent based on per cent tear values.

Example I Instead of the 30 pounds of chlorine conventionally suppliedto partially bleach this pulp in the preliminary chlorination stage, 10pounds of sodium chlorite together with 15 pounds of chlorine, 3 ofwhich were necessary to activate the chlorite, were used. After thetreatment had continued for one-half hour the pulp was filtered andwashed. The caustic extraction was omitted and the pulp at a consistencyof 12 per cent was treated with 28 pounds of available chlorine ascalcium hypochlorite at a temperature of F. for two hours. test sheetsshowed a GE brightness of 80.7. The strength After washing,

retention after completion of the treatment was 112 per cent based onthe per cent Mullen values which were 182 for the unbleached stock and204 for the final pulp. Based on the per cent tear values for theunbleached stock of 171 and on the final pulp of 179 the strengthretention was 104.6 per cent. By this operation a saving of 21 pounds ofchlorine per ton of pulp or 32.8 per cent of the chlorine normallyrequired was effected.

Example II The same pulp at a consistency of 2.5 per cent was treatedwith 20 pounds of available chlorine as sodium chlorite and pounds ofchlorine per ton, 4 of which were utilized to activate the chlorite.After treatment in the preliminary chlorination stage for one-half hourthe pulp was filtered, washed and reslurried to a consistency of 10 percent with pounds of caustic per ton. The caustic extraction wascontinued at a temperature of 140 F. for one hour. After washing thepulp and reslurrying to a consistency of 12 per cent, it was treatedwith 9 pounds of available chlorine as calcium hypochlorite at atemperature of 95 F. for two hours. The finished pulp gave a test sheethaving a brightness of 83.9. The strength retention was 92.8 per centbased on per cent Mullen values of 184 for the unbleached stock and 171for the final pulp. Based on per cent tear values the strength retentionwas 111.5 per cent. The chlorine requirement in the first stage wasreduced by 70 per cent of that when no chlorite was used and the totalavailable chlorine requirement was reduced by per cent.

Example 111 to per cent of the chlorine requirement of the pulp asdetermined by the permanganate number is supplied by chlorine followedby a later stage of bleaching with an aqueous solution of hypochlorite,the improvement which comprises partially bleaching the pulp in thepreliminary stage in aqueous medium and under acid conditions withchlorine dioxide and chlorine, the amount of the chlorine requirementsupplied to the preliminary partial bleaching stage by the chlorinedioxide being about 50 to per cent on an equivalent available chlorinebasis.

2. In the multi-stage process of bleaching sulfite cellulose pulps inwhich the pulp to be bleached is subjected in a preliminary stage to apartial bleaching in aqueous medium in which about 50 to 60 per cent ofthe chlorine requirement of the pulp as determined by the permanganatenumber is supplied by chlorine followed by a later stage of bleachingwith an aqueous solution of hypochlorite, the improvement whichcomprises partially bleaching the pulp in aqueous medium at a pH ofabout 2.5 to 2.7 at approximately room temperature with a chlorite andchlorine, the amount of the chlorine requirement supplied to thepreliminary partial bleaching stage by the chlorite being about 50 to 75per cent on an equivalent available chlorine basis.

References Cited in the file of this patent UNITED STATES PATENTS2,129,719 Vincent Sept. 13, 1938 2,166,330 Vincent July 18, 19392,235,837 Logan Mar. 25, 1941 FOREIGN PATENTS 191,357 Great Britain Mar.22, 1923 446,444 Belgium July 15, 1942 OTHER REFERENCES Woodside et al.:Condensation of Round Table Discussion on Pulp Bleaching. Paper TradeJournal, December 2, 1948, pgs. 21 to 25, especially at pg. 24, col. 2.

Sen Gupta: Progressive Delignification of Jute Fibre with ChlorineDioxide. J. Text. Inst. (Trans), September 1951, pgs. T375-384,especially first page.

1. IN THE MULTI-STAGE PROCESS OF BLEACHING SULFITE CELLULOSE PULPS INWHICH THE PULP TO BE BLEACHED IS SUBJECTED IN A PRELIMINARY STAGE TO APARTIAL BLEACHING IN AQUEOUS MEDIUM AND UNDER ACID CONDITIONS IN WHICHABOUT 50 TO 60 PER CENT OF THE CHLORINE REQUIREMENT OF THE PULP ASDETERMINED BY THE PERMANGANATE NUMBER IS SUPPLIED BY CHLORINE FOLLOWEDBY A LATER STAGE OF BLEACHING WITH AN AQUEOUS SOLUTION OF HYPOCHLORITE,THE IMPROVEMENT WHICH COMPRISES PARTIALLY BLEACHING THE PULP IN THEPRELIMINARY STAGE IN AQUEOUS MEDIUM AND UNDER ACID CONDITIONS WITHCHLORINE DIOXIDE AND CHLORINE, THE AMOUNT OF THE CHLORINE REQUIREMENTSUPPLIED TO THE PRELIMINARY PARTIAL BLEACHING STAGE BY THE CHLORINEDIOXIDE BEING ABOUT 50 TO 75 PER CENT ON AN EQUIVALENT AVAILABLECHLORINE BASIS.